SUGGESTION: set_slot

{
    "pools": [
        {
            "rolls": 1,
            "entries": [
                {
                    "type": "item",
                    "name": "minecraft:stone",
                    "weight": 1,
                    "functions": [
                        {
                            "function": "set_slot",
                            "slot": 3
                        }
                    ]
                },
                {
                    "type": "item",
                    "name": "minecraft:diamond",
                    "weight": 1,
                    "functions": [
                        {
                            "function": "set_slot",
                            "slot": {
                                "min": 1,
                                "max": 9
                            }
                        }
                    ]
                }
            ]
        }
    ]
}